1. Field of the Invention
The invention relates to a spark ignition internal combustion engine in which a high-octane fuel and a low-octane fuel are mixed by a fuel mixture means adapted for mixing high and low-octane fuels at variable mixing ratios and a mixed fuel is supplied into a combustion chamber.
2. Description of the Related Art
Low-octane fuels are good in ignitability but poor in antiknock property, whereas high-octane fuels are poor in ignitability but good in antiknock property. Internal combustion engines in which a low-octane fuel and a high-octane fuel are mixed and supplied to an engine by fuel injection valves so as to meet an operating condition of the engine are known, as described in Japanese Patent Application Laid-Open Publication No. 2001-050070.
In such an internal combustion engine, however, the octane number of a mixed fuel supplied into the combustion chamber sometimes deviates from a target octane number due to the tolerance of a mixing device of the fuel injection valve or the like, although the high-octane fuel and the low-octane fuel respectively have predetermined octane numbers. According to the existing technologies, there is no method for detecting this deviation in octane number. Therefore, if the octane number deviates, the engine is operated with the deviated octane number remaining, and therefore cannot exactly deliver intended operation performance, that is, intended accelerating performance, intended fuel consumption, and intended exhaust emissions.
Therefore, in an apparatus described in Japanese Patent Application Laid-Open Publication No. 4-234571, the octane number of fuel is estimated, and an ignition timing corresponding to the estimated octane number is set so that the engine can always operate at a knocking limit. However, the apparatus described in Japanese Patent Application Laid-Open Publication No. 4-234571 merely performs a generally-termed symptomatic treatment in which a deviation in the octane number of fuel is met by adjustment in the ignition timing, and does not provide a radical cure in which the octane number of fuel is set to a predetermined value. That is, since the mixing proportion is not determined, the apparatus is not able to eliminate deviation in the mixing proportion. In the case of a large deviation in the mixing proportion, the apparatus may be incapable of achieving a practical effect.